drivers: ieee802154: cc13/26xx_subg: fix header len const

The length field in the header refers to the size of the MAC so it
shouldn't rely on constants describing PHY header length. While
currently both constants have the same value this will no longer be true
for enhanced PHYs and/or MAC frames as the number of FCS bytes may then
be four.

Also introduces an assertion that ensures that the given package buffer
does not exceed the TX buffer's length. An assertion is enough as the
package buffer is allocated at compile time.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>